Towards a vaccine for rheumatic fever: identification of a conserved target epitope on M protein of group A streptococci

New multi-determinant strategy for a group A streptococcal vaccine designed for the Australian Aboriginal population

Infection with group A streptococci can result in acute and post-infectious pathology, including rheumatic fever and rheumatic heart disease. These diseases are associated with poverty and are increasing in incidence, particularly in developing countries and amongst indigenous populations, such as Australia's Aboriginal population, who suffer the highest incidence worldwide. Immunity to group A streptococci is mediated by antibodies against the M protein, a coiled-coil alpha helical surface protein of the bacterium. Vaccine development faces two substantial obstacles. Although opsonic antibodies directed against the N terminus of the protein are mostly responsible for serotypic immunity, more than 100 serotypes exist. Furthermore, whereas the pathogenesis of rheumatic fever is not well understood, increasing evidence indicates an autoimmune process. To develop a suitable vaccine candidate, we first identified a minimum, helical, non-host-cross-reactive peptide from the conserved C-terminal half of the protein and displayed this within a non-M-protein peptide sequence designed to maintain helical folding and antigenicity, J14 (refs. 8,9). As this region of the M protein is identical in only 70% of group A streptococci isolates, the optimal candidate might consist of the conserved determinant with common N-terminal sequences found in communities with endemic group A streptococci. We linked seven serotypic peptides with J14 using a new chemistry technique that enables the immunogen to display all the individual peptides pendant from an alkane backbone. This construct demonstrated excellent immunogenicity and protection in mice.

Epidemiological analysis of non-M-typeable group A Streptococcus isolates from a Thai population in northern Thailand

Infection with group A streptococci (GAS) can lead to the development of severe postinfectious sequelae such as rheumatic fever (RF). In Thailand, RF and rheumatic heart disease (RHD) remain important health problems. More than 80% of GAS circulating in this population are non-M antigen typeable by conventional M serotyping methods. In this study, we determine the M protein sequence types of GAS isolates found in northern Thailand. The emm genes from 53 GAS isolates, collected between 1985 and 1995 from individuals with pharyngitis, impetigo, acute RF (ARF), RHD, or meningitis as well as from individuals without infections, were amplified by PCR and sequenced. Thirteen new sequence types that did not show homology to previously published sequences were characterized. Six of these sequence types could be isolated from both skin and throat sites of impetigo and pharyngitis/ARF patients, respectively. In many cases we could not specifically differentiate skin strains or throat strains that could be associated with ARF or acute glomerulonephritis. Antigenic variations in the emm gene of the isolates investigated, compared to published M protein sequences, were predominantly due to point mutations, small deletions, and insertions in the hypervariable region. One group of isolates with homology to M44 exhibited corrected frameshift mutations. A new M type isolated from an RHD patient exhibited nucleotide sequence corresponding to the N terminus of M58 and the C terminus of M25, suggesting that recombination between the two types may have occurred. This study provided epidemiological data relating to GAS endemic to northern Thailand which could be useful for identification of vaccine candidates in a specific region of endemicity.

Acute rheumatic fever in Konya, Turkey

BACKGROUND

Patients with acute rheumatic fever (ARF), who were admitted to Pediatric Cardiology Unit of Selçuk University Faculty of Medicine from July 1993 to 1998, were studied retrospectively to verify the clinical profile of the disease and to compare the results with those from other countries.

METHODS

All patients were examined by one of the two pediatric cardiologists in our study group. Every patient had a chest X-ray, electrocardiogram and an echocardiographic investigation. Throat culture, antistreptolysin O test, C-reactive protein and sedimentation rates were investigated for each patient.

RESULTS

During the study period, 274 cases with ARF were identified among patients admitted to the present institution. There were 8032 visits during the study period, giving an occurrence rate of 3.4%. Arthritis was the most common major manifestation (81.4%). It was followed by carditis (60.9%) and chorea (17.9%). Subcutaneous nodules (0.7%) and erythema marginatum (0.4%) were both seen in patients with carditis. The mitral valve was the most commonly affected valve (95.8%), followed by the aortic valve (40.1%). Two patients died and regurgitation disappeared in 21% of patients with mitral regurgitation. Fifteen patients (14%) with isolated arthritis and pure chorea had mitral regurgitation demonstrated by echocardiographic investigation but without any significant murmur.

CONCLUSION

The present study indicates that ARF is still a significant problem in Konya and that recurrences can be prevented by administering a 3-week benzathine penicillin G regimen.

Functional analysis of IgA antibodies specific for a conserved epitope within the M protein of group A streptococci from Australian Aboriginal endemic communities

The mucosa is one of the initial sites of group A streptococcal (GAS) infection and salivary IgA (sIgA) is thought to be critical to immunity. However, the target epitopes of sIgA and the function of sIgA in GAS immunity, in particular the role of accessory cells and complement, is largely unknown. We studied the aquisition and the function of sIgA specific for a conserved region epitope, p145 (sequence: LRRDLDASREAKKQVEKALE) of the M protein. Peptide 145-specific sIgA is highly prevalent within an Aboriginal population living in an area endemic for GAS and acquisition of p145-specific sIgA increases with age, consistent with a role for such antibodies in immunity to GAS. Human sIgA and IgG specific for p145 were affinity purified and shown to opsonize M5 GAS in vitro. Opsonization could be specifically inhibited by the addition of free p145 to the antibodies during assay. Opsonization of GAS was totally dependent on the presence of both complement and polymorphonuclear leukocytes, and, moreover, affinity-purified p145-specific sIgA was shown to fix complement in the presence of M5 GAS. These data show that mucosal IgA to this conserved region peptide within the M protein has an important role in human immunity against GAS and may be useful in a broad-based cross-protective anti-streptococcal vaccine.

Vaccine strategies to prevent rheumatic fever

Group A streptococci (GAS) are responsible for numerous human illnesses, ranging from pharyngitis to severe invasive infections, such as necrotizing fascitis and toxic shock syndrome to the postinfectious sequelae, acute rheumatic fever (ARF), and glomerulonephritis. To date, to develop a vaccine, studies have focused on the M protein. However, designing a vaccine to prevent GAS infection based on this molecule has been hampered by the vast number of M protein serotypes and the possibility that it may induce potentially harmful autoimmune reactions. In this article, the authors discuss recent approaches to overcoming the problems of an M protein-based vaccine. In addition, recent studies identifying the protective properties of other streptococcal antigens and their potential as vaccine candidates are discussed.

Human antibodies to the conserved region of the M protein: opsonization of heterologous strains of group A streptococci

A 20-mer peptide (p145) in the carboxyl-terminal region of the M protein of group A streptococci (GAS) has previously been defined as the target of bactericidal antibodies. Sequence analysis of seven field isolates from indigenous Australians living in an area highly endemic for GAS and five laboratory reference strains (encompassing nine unique serotypes plus three nontypeables) demonstrates that this region is highly conserved (sequence identity ranging from 65 to 95%) with six of the 12 sequences being identical to p145. Most of the sequence dissimilarity is contained within the last seven amino acids of p145. Competitive ELISA demonstrates that human antibodies specific for p145 cannot discriminate between p145 and synthetic peptides representing four from four of the variant sequences tested. Ig purified from endemic sera was able to opsonize each of the GAS isolates and free p145 as well as a peptide expressing a minimal conformational epitope within p145 (requiring amino acids between positions 2 and 13 of p145), but not an irrelevant peptide, were able to partially or completely inhibit opsonization of all isolates and reference strains. Thus adult endemic sera contain antibodies which are bactericidal for multiple GAS serotypes and which are specific for a sequence of 12 amino acids contained within the p145 region of the M protein.

Overview of vaccines

This article lists the vaccines current available for the control of both viral and bacterial infections. They may be attenuated live or inactivated whole microorganisms, or subunit preparations. Many more are in the pipeline and increasing attention is being given to establishing their safety before registration. Following the earlier eradication of smallpox, good progress is now being made toward the global eradication of poliomyelitis and a new program to eliminate measles from the Americas has begun. A variety of new approaches to vaccine development is now available. The hepatitis B virus surface antigen, made by DNA-transfected yeast or mammalian cells, is the basis of the first genetically engineered vaccine. Early in the 21st century, new vaccines based on oligopeptides, recombinant live viral or bacterial vectors (often existing live vaccines), or recombinant DNA plasmids are likely to be registered for human use. The efficacy of vaccines depends on the immune responses generated, and the recent substantial increase in our understanding of the mammalian immune system now offers great opportunities for manipulation to best obtain desired responses. These include mixing vaccine formulations to maximize immune responses, and combining vaccines to simplify their administration. Despite these advances, some persisting infections, such as those caused by HIV, plasmodia, and mycobacteria, still pose a great challenge to vaccine developers.

Acute rheumatic fever. Still a challenge

At the end of the 20th century, after an apparent decline, acute rheumatic fever (ARF) now constitutes a great challenge for developed and developing countries. It is caused by a group A beta-hemolytic Streptococcus upper airways infection, but the exact pathogenetic mechanisms are not yet clear. The role of the immune system in the pathogenesis of ARF is understood better than genetic host factors. ARF can mimic many other diseases, and the diagnosis is based on clinical criteria. It is still overdiagnosed and underdiagnosed in different settings. Penicillin has greatly contributed to the reduction in the incidence and recurrence of this disease. Current schemes of prophylaxis, however, present many problems, and failures are common. Future efforts to reduce the burden of this disease should induce public health measures the vaccine strategies.

Case study: an infection-triggered, autoimmune subtype of anorexia nervosa

OBJECTIVE

Certain cases of anorexia nervosa (AN) may be similar to the recently described subtype of childhood-onset obsessive-compulsive disorder hypothesized to be one of the pediatric infection-triggered autoimmune neuropsychiatric disorders (PITANDs).

METHOD

Three clinical cases are reported. The first patient is a 12-year-old boy whose AN worsened acutely after a group A beta-hemolytic streptococcal (GABHS) infection. His symptoms were alleviated after antibiotic treatment. Two other patients with possible PITANDs-related AN are described.

RESULTS

An infection-triggered process may contribute to the pathogenesis of a subtype of AN.

CONCLUSIONS

Future research is needed to explore the nature of PITANDs and their relationship with AN.

Mapping of conformational B cell epitopes within alpha-helical coiled coil proteins

An approach to mapping antigenic B cell epitopes within alpha-helical coiled coil proteins has been developed and applied to two proteins: Streptococcal M protein and C. elegans paramyosin protein UNC-15. Overlapping peptides derived from an alpha-helical coiled coil conformational epitope were embedded between helical flanking peptides derived from the completely unrelated GCN4 leucine zipper peptide. The resulting chimeric peptides exhibited helical propensity. Chimeric peptides were tested for antigenicity (recognition by antibody) or immunogenicity (production of appropriate antibody response). A conformational epitope within the Streptococcal M protein recognised by three mAbs spanned 12 residues. Analysis of chimeric peptides based on C. elegans UNC-15 has enabled fine mapping of the minimal B cell epitope recognised by monoclonal antibody NE1-6B2 to seven non-contiguous residues (spanning 15 residues); the footprint of contact residues involved in antibody recognition being restricted to the hydrophilic face of the helix and covering five helical turns. This chimeric peptide epitope when coupled to diphtheria toxoid was highly immunogenic in mice and antisera recognised the conformationally dependent native peptide epitope. This approach has the potential to map conformational epitopes and design minimal epitopes for use as vaccine candidates.

Antigenic epitope mapping of the M24 protein of Streptococcus pyogenes: implications for serodiagnosis of rheumatic fever

Rheumatic fever continues to be a significant problem in Australian Aboriginal communities and developing countries worldwide. Early diagnosis could facilitate the institution of penicillin prophylaxis resulting in the prevention of recurrences of rheumatic fever. An overlapping biotinylated peptide bank of 82 peptides, based on the known sequence of Streptococcus pyogenes M24 protein, was used in a standard enzyme immunoassay. A total of 82 sera were tested from both aboriginal and non-aboriginal subjects with clinically proven rheumatic fever, rheumatic heart disease and matched controls. Two peptides with significant sequence homology at the C-terminal end were found to be discriminatory between aboriginal cases and controls. It is proposed that these peptides could be the basis of a serological test for rheumatic fever.

Opsonic human antibodies from an endemic population specific for a conserved epitope on the M protein of group A streptococci

This study demonstrates the presence of epitope-specific opsonic human antibodies in a population living in an area endemic for group A streptococci (GAS) infection. Antibodies recognizing a conserved C-terminal region epitope (p145, sequence in single letter amino acids: LRRDLDASREAKKQVEKALE) of the M protein of GAS were isolated from human patients by affinity chromatography and were shown to be of the immunoglobulin G1 (IgG1) and IgG3 subclasses. These antibodies could reduce the number of colonies of serotype 5 GAS in an in vitro opsonization assay by 71-92%, compared with an equal amount of IgG from control adult donors living in non-endemic areas and without antibodies to p145. Addition of the peptide, p145, completely inhibited this opsonization. Indirect immunofluorescence showed that p145-specific antibodies were capable of binding to the surface of M5 GAS whereas control IgG did not. Using chimeric peptides, which contain overlapping segments of p145, each 12 amino acids in length, inserted into a known helical peptide derived from the DNA binding protein of yeast, GCN4, we have been able to further define two minimal regions within p145, referred to as pJ2 and pJ7. These peptides, pJ2 and pJ7, were able to inhibit opsonization by p145 specific antibodies. Finally, we have observed an association between the age-related development of immunity to GAS and the acquisition of antibodies to the conserved epitope, p145, raising the possibility of using this epitope as a target in a prophylactic vaccine administered during early childhood.

Non-specific serum binding to streptavidin in a biotinylated peptide based enzyme immunoassay

Streptavidin coated microtitre wells can be used to provide attachment to biotinylated peptides, regardless of charge and size, for use in an enzyme immunoassay. Considerable non-specific human serum binding to streptavidin at the recommended concentration was noted. The level of binding varied considerably with serum samples and was unrelated to previous streptococcal disease. Strategies used to reduce this phenomenon included pre-absorption of serum with streptavidin and reducing the streptavidin concentration in the wells. Significant reduction in binding was found with a reduction in streptavidin concentration from the recommended concentration of 0.5 ug/well to 0.01 ug/well.

Towards understanding the pathogenesis of rheumatic fever

Acute rheumatic fever results from an immunological response to group A streptococcal infection, but the exact nature of this response, and of the underlying host and organism characteristics, continues to evade researchers. Earlier models of rheumatic fever pathogenesis emphasised the importance of humoral immunity, but more recent work suggests that cellular immunity may play a primary role. Greater understanding of these disease mechanisms is allowing researchers to move towards the development of a vaccine for rheumatic fever.